A reduced projection exposure apparatus (or a photolithography machine) is mainly used for transferring patterns of an integrated circuit (IC), a large-scale integrated circuit (LSI), and the like. Along with an increase in the degree of integration of an integrated circuit, a projection optical system used in such a machine is required to possess a wider exposure area and a higher resolution throughout the exposure area. As a method of improving the resolution of the projection optical system, it is conceivable to reduce an exposure wavelength or to increase a numerical aperture (NA) of the projection optical system.
In terms of the exposure wavelength, reduction in the wavelength is now in progress by making the shift from a KrF (248 nm) excimer laser currently used as a main light source for a semiconductor exposure apparatus to an ArF (193 nm) excimer laser which is a deep ultraviolet light source. Moreover, to achieve even higher integration in the future, an F2 (157 nm) laser which is vacuum ultraviolet light is now being studied.
Recently, a microchannel phenomenon representing a mechanical damage on an optical component is drawing attention. The microchannel phenomenon means a phenomenon to form a void having a diameter of several micrometers inward from a surface of an optical component (E. M. Wright et al., “Spatial pattern microchannel formation in fused silica irradiated by nanosecond ultraviolet pulses”, Applied Optics, 1999, Vol. 38. p. 5785-5788).
When a microchannel is formed, light transmission of a component may be reduced by light scattering or an optical thin film may be destroyed. Accordingly, transmission property of the optical component is considerably deteriorated. The microchannel is formed by continuously irradiating some 1 billion (109) pulses of an ArF excimer laser having energy density in a practical range of several millijoules per square centimeters onto synthetic silica glass, for example. For this reason, it is necessary to implement a measure for preventing mechanical damages attributable to formation of microchannels even under normal service conditions of an exposure apparatus.
The inventors of the present invention have found out the fact that formation of the microchannel is closely related to an increase in density of silica glass and have filed a related patent application (Japanese Patent Application No. 2003-38345). The increase in density of silica glass is apt to occur more frequently as the wavelength of an irradiated beam becomes shorter. Accordingly, the microchannel becomes a serious problem as an exposure apparatus is equipped with higher resolution. Moreover, since the density of silica glass is increased without exception by irradiation of an ultraviolet laser having a high energy density, microchannels are formed in almost any kinds of silica glass when the laser exceeds a certain threshold of irradiation energy. Therefore, one conceivable measure for preventing formation of microchannels is to limit an irradiation energy density to silica glass. However, a given energy density is necessary on a resist surface in terms of a reduced projection exposure apparatus. Accordingly, there may be a case where it is not possible to suppress an output from a light source due to a design reason.
Another solution is a method of applying a transparent crystalline material, which does not cause an increase in density, to an optical component subject to irradiation of a light beam having a high energy density instead of applying silica glass. By using this method, it is possible to prevent formation of microchannels in an optical component without suppressing an output from a light source.
Industrially applicable crystalline materials having high transmittance and high chemical stability in a short wavelength range such as an excimer laser are limited. Such materials include fluorite and crystalline quartz, for example.
Fluorite (CaF2) is an excellent optical material which has a cubic crystal, optical isotropy, and an optical band gap approximately equal to 12 eV (transparent up to about 100 nm). Fluorite is used as a material of a lens in a reduced projection exposure apparatus applying an ArF excimer laser as a light source. However, fluorite also possesses unfavorable features of cleavage on the (111) plane and resultant mechanical weakness. For example, fluorite is ranked at 4 on the modified Mohs hardness scale (defining 15 levels). Accordingly, fluorite has a problem of easily causing damages and the like and it is difficult to form this material into an optical element.
Meanwhile, crystalline quartz (the molecular formula: SiO2) has a hexagonal crystal and an optical band gap thereof is estimated to be around 9 eV. Crystalline quartz is not cleaved and is therefore mechanically strong. For example, crystalline quartz is ranked at 8 on the modified Mohs hardness scale (defining 15 levels). Accordingly, crystalline quartz has superior workability to fluorite.
Nevertheless, crystalline quartz has an anisotropic crystal structure and thereby exhibits strong birefringence. For this reason, crystalline quartz is applicable to an optical member not harmed by such birefringence or to an optical member configured to actively utilize the birefringence of crystalline quartz.
In these circumstances, it is proposed to use crystalline quartz as a part of illumination system optical components in a reduced projection exposure apparatus. Japanese Patent Application Laid-Open Gazette No. Hei 5(1993)-47636 discloses a technique to actively utilize the birefringence of crystalline quartz to fabricate a polarizing beam splitter or a depolarizer using crystalline quartz. The birefringence is essential to fabrication of these optical elements. Accordingly, fluorite or silica glass is not applicable thereto.
Meanwhile, Japanese Patent Application Laid-Open Gazette No. 2002-75835 discloses a technique to apply a diffractive optical element including a special process on a surface of a substrate thereof or a micro fly eye lens to a homogenizer for homogenizing a beam. It is extremely difficult to fabricate these optical elements by using fluorite because of the inadequate mechanical strength. Moreover, as the homogenizer is used in a position close to a light source, an energy density of an irradiated beam is relatively high. Accordingly, there is a risk of formation of microchannels when the homogenizer is made of silica glass. For this reason, it is preferable to apply crystalline quartz, which is mechanically strong and capable of eliminating formation of microchannels, to these optical elements.